#include <fstream>
#include <vector>
using namespace std;
ifstream fin ("mesaj4.in");
ofstream fout("mesaj4.out");
int fiu,n,i,m,x,j,k,y,maxim;
bool viz[100003];
vector <int> L[100003];
struct elem
{
    int x,y;
};
vector <elem> sol;
void dfs(int x)
{
    viz[x]=1;
    for(int j=0;j<L[x].size();j++)
        if(!viz[L[x][j]])
        {
            dfs(L[x][j]);
            sol.push_back({L[x][j],x});
        }
}
int main()
{
    fin>>n>>m;
    for(i=1;i<=m;i++)
    {
        fin>>x>>y;
        L[x].push_back(y);
        L[y].push_back(x);
    }
    k=0;
    for(i=1;i<=n;i++)
        if(!viz[i])
        {
            k++;
            dfs(i);
        }
    if(k!=1)
    {
        fout<<-1;
        return 0;
    }
    fout<<2*n-2<<"\n";
    for(j=0;j<sol.size();j++)
        fout<<sol[j].x<<" "<<sol[j].y<<"\n";
    while(!sol.empty())
    {
        fout<<sol.back().y<<" "<<sol.back().x<<"\n";
        sol.pop_back();
    }
    return 0;
}